1. Field of the Invention
The present invention relates to the provision of device isolation regions between devices of an integrated circuit formed in and on a semiconductor substrate.
2. Description of the Related Art
A variety of integrated circuits incorporate device isolation regions formed between adjacent semiconductor devices to prevent carriers from traveling through the substrate between the adjacent devices. For example, device isolation regions are conventionally formed between adjacent field effect transistors (FETs) in dense semiconductor circuits such as dynamic random access memories (DRAMs) to reduce charge leakage to and from the FETs. Often, device isolation regions take the form of thick field oxide regions extending below the surface of the semiconductor substrate. The most common early technique for forming a field oxide region is the local oxidation of silicon ("LOCOS") technique, the results of which are schematically illustrated in FIG. 1. Referring to FIG. 1, a silicon substrate 10 has a field oxidation region 12 separating two FETs 14 and 16. Each of the FETs has a source, drain, channel, and a gate over the channel region. The illustrated devices might be part of a memory circuit, and there would conventionally be additional circuitry included for this portion of the circuit including, for example, charge storage capacitors, wiring lines formed on the field oxide region 12 and over other regions, and a variety of other wiring lines, contacts and circuits.
LOCOS field oxidation regions are formed by first depositing a layer of silicon nitride on the substrate surface and then selectively etching a portion of the silicon nitride layer to form a mask exposing the substrate where the field oxidation will be formed. The masked substrate is placed in an oxidation environment and a thick silicon oxide layer is grown at the regions exposed by the mask, forming an oxide layer extending above and below the surface of the substrate. The silicon nitride mask is almost unaffected by the oxidation environment and the mask limits lateral oxidation, so that the lateral extent of the field oxidation region is largely determined by the mask. Oxide growth undercuts the silicon nitride mask at its edge, however, extending laterally into and somewhat above the substrate along the edge of the nitride mask. Consequently, the field oxide region 12 includes what are known as "bird's beak" regions 18 along its edges. As illustrated in FIG. 1, the bird's beak regions 18 are thinner oxide regions extending away from the main portion of the field oxidation region along the surface of the substrate.
The bird's beak regions formed in the LOCOS growth of field oxide region 12 are undesirable, particularly as further attempts are made to reduce the size and spacing of memory cells. Because the bird's beak regions are thin, they provide little in terms of device isolation. The bird's beak regions nevertheless consume substrate surface area, limiting the extent to which the field oxide region can be shrunk while still providing desirable levels of device isolation. To provide higher device densities, it is therefore desirable to utilize a different device isolation structure.
An alternative to LOCOS field oxidation is the use of shallow trench isolation ("STI"). In STI, a sharply defined trench is formed in the semiconductor substrate by, for example, anisotropic etching. The trench is filled with oxide back to the surface of the substrate to provide a device isolation region. Trench isolation regions formed by STI have the advantages of providing device isolation across their entire lateral extent and of providing a more planar structure. Aspects of the manufacture of conventional trench isolation regions, however, may reduce the reliability of structures and devices formed immediately adjacent to the trench isolation region.
It is accordingly an object of the present invention to provide a trench isolation region that provides desirable levels of reliability in the structures and devices formed adjacent to the trench isolation region as well as in the level of isolation provided by the trench isolation.